Yes, you should seal the stringers before installing with 2-3 coats of epoxy. Once cured, sand the side to be bonded to the hull, sand the hull where they will be installed. Mix some epoxy and brush some unfilled onto the hull sanded hull side. Thicken the remaining with 406 colloidal silica to a mayonnaise consistency, brush this paste onto the stringers, and secure the srtingers in place until the epoxy cures. You may be able to use screws to temporarily hold the stringers against the hull, then fill the screw holes once they are removed. Be sure not to tighten the stringers too tightly and squeeze out all the epoxy.